Phthalic anhydride (as well as phthalic acid, from which the anhydride is easily prepared) derivatives bearing a reactive group in the 4-position are useful intermediates for the production of high-performance polyimides, which are used in many applications, such as composites, foams, molded articles and microelectronic coatings. These polymers are very useful because of their excellent UV resistance, thermaloxidative stability, chemical resistance and flame retardancy. Polyetherimides have been prepared by nitro-displacement of nitrophthalic anhydride [D. M. White et al., J. Pol. Science 19, 1635 (1981)] as well as by chlorine or fluorine displacement of the corresponding halogenated phthalic derivatives [H. M. Relles, Contemp. Top. Polym. Sci. 5, 261 (1984)]. The 4-halo-derivatives also find use in the manufacture of 4,4'-oxydiphthalic anhydride and of 4,4'-biphenyltetracarboxylic dianhydride, which are combined with diamines to produce heat-resistant polyimide plastics. U.S. Pat. No. 4,697,023 discloses the use of the bromo-derivative, along with chloro- and flouoro- derivatives for the production of oxy-diphthalic anhydrides.
A severe problem, to which the art has so far been unable to provide a solution, is that in the manufacture of 4-substituted phthalic acids and anhydrides the 3-isomers and several disubstituted derivatives are usually produced as undesired by-products. Furthermore, often unreacted phthalic anhydride or acid remains in the final product. This problem exists in all known preparation processes. For instance, the preparation of 4-bromophthalic acid (which will be hereinafter called 4-BrPA, for the sake of brevity) or the anhydride, which are particularly useful intermediates, can be prepared in a number of ways. For example, phthalic acid has been reacted with potassium bromate, to provide a yield of 50% [JACS 46,2169,1981] or with a mixture of an alkali metal bromide and an oxidizing agent such as hypochlorite or hypobromite, or phthalic acid (hereinafter PA) has been brominated in 60% oleum. Metal salts of PA have been brominated in water directly with bromine [U.S. Pat. No. 2,394,268] or in a non-protonic polar solvent, as in Japanese Patent No. 79-29348. In these processes, the product is contaminated to varying degrees with the 3-BrPA isomer, and with di- and higher brominated impurities, depending on the brominating system and conditions employed. In an alternative process [see, e.g., CA 90: 38640e, 1979], brominated xylenes may be oxidized, but 4-bromoxylene is usually contaminated with other mono-brominated xylenes, as well as with higher brominated materials.
Removing the contaminants obtained in the processes of the art is very difficult because of the great similarity of the physical properties of the impurities with those of the desired product, viz., the mono-substituted 4-derivative. For this reason, separating the contaminants from the final product has been industrially impractical. As a result of this fact, substituted PAs normally used are not highly pure, the contents and types of contaminants vary from one batch of material to another, which lead to erratic results and performances of the plastics into which they are incorporated.
It is therefore clear that it would be highly desirable to be able to prepare 4-halo-phthalic acids which are highly pure, by an industrially applicable process. It has now surprisingly been found that it is possible selectively to precipitate 4-mono-halogenated phthalic acids from aqueous solutions containing contaminants, and to prepare highly pure final products, with purities of 99% or higher.
It has further been most surprisingly found that when 4-halo-phthalic acid is precipitated from the aqueous solution, under conditions which will be detailed hereinafter, a novel material is formed, which was so far unknown in the art, which is a bis(4-halo-phthalic acid) quarter salt, viz., a material which is obtained by the formation of a complex of the 4-halophthalic acid with a mono- salt of the diacid with an alkali metal or with ammonium.
It is therefore an object of the present invention to provide novel compounds, which can be termed "quarter salts". It is another object of the present invention to provide a process for preparing such quarter salts.
It is a further object of the invention to provide a process for preparing 4-halo-phthalic acids in very high purity, which is simple, inexpensive and industrially applicable.
It is still another object of the invention to provide a method for purifying 4-halo-phthalic acids which contain impurities, particularly impurities which are not simply separated from the desired halo-substituted phthalic acid.